3.1. Proximate Composition
The passion fruit flour (PFF), which was elaborated with non-edible parts of the fruit (peels and seeds) showed amounts of proteins, lipids, and fiber higher than that of the passion fruit pulp (Table 1
). According to Morais et al. [3
], peels and seeds of tropical fruits usually present higher amounts of nutrients, such as proteins, fiber, lipids, and minerals, than their pulps, which demonstrate their nutritional relevance for human nutrition. The main component of PFF was the dietary fiber (64.0%) with insoluble fiber presenting the largest contribution (50.4%) as compared to soluble fiber (13.6%). Insoluble fiber plays an essential role in promoting regular bowel movement, which prevents constipation [33
]. Fiber intake has also been correlated with the management of diabetes and the prevention of cardiovascular diseases, weight gain, and colorectal cancer [34
The lipid and protein contents of PFF were 9.9% and 8.7%, respectively. The ash content indicated that PFF might be a good source of minerals. The previous report about PFF indicated 5.8–7.2% for moisture, 5.2–7.1% for ash, 7.9–10% for protein, 42–57% for dietary fiber, and 0.7–5.64% for lipids in its composition [3
]. These results are in accordance with the present study, except for the lipid content. Seeds are the main source of fatty acids in PFF, and their absence or smaller amount can interfere with the total lipid content [3
]. Other factors, such as fruit variety, soil, and climate conditions for passion fruit production, and distinct formulations of the flour may also affect the PFF composition.
The addition of PFF resulted in a concurrent decrease in drinkable yogurt moisture, which was already expected due to the hygroscopic capacity of the fiber present in the passion fruit flour (64%). When compared to the commercial yogurt, all treatments showed lower moisture values. The ash content increased in the drinkable yogurts due to the addition of PFF; this is supported by its high ash and mineral content (Table 1
). Likewise, the lipid content increased with the addition of PFF, which was likely due to the seeds. According to Morais et al. [3
], the lipid fraction present in passion fruit seeds renders a high content of unsaturated fatty acids from which linoleic (16%) and oleic acids (4.2%) are the major compounds. Unsaturated fatty acids are recognized for their potential to lower the risk of heart disease [37
Traditionally, yogurts do not contain a significant amount of fiber. The samples of plain drinkable yogurt, commercial yogurt, and drinkable yogurt 0%, which did not present PFF in their composition, showed fiber contents lower than 0.5%. On the other hand, drinkable yogurts with PFF showed a higher fiber content, which was in the range of 1.6–5.3%. These results are supported by the proximate composition of PFF, which evidenced its role as a rich source of soluble and insoluble fiber and its great potential to increase the nutritional value of drinkable yogurts and other food products [18
3.2. Mineral Content
In general, yogurts containing passion fruit pulp and flour showed a higher mineral content than the commercial formulation (Table 1
). The addition of PFF and pulp in drinkable yogurt was able to increase the amount of phosphorus (from 13.5 to 17.7%), potassium (from 2.8 to 6.7%), calcium (from 9.5 to 16.4%), magnesium (from 2.5 to 4.6%), iron (from 6.2 to 8%), and zinc (from 7.5 to 10%). Additionally, according to the recommendations of the National Institutes of Health [38
], the intake of 100 g of drinkable yogurt containing PFF (yogurt 2–8%) would provide 16.2–17.7% of phosphorus, 11.6–16.4% of calcium, 4.8–6.7% of potassium, 3.4–5.8% of sodium, and 2.6–5% of magnesium recommended daily intake (RDI) for men and women, respectively, between 19 and 70 years. Considering that minerals should be provided not only by yogurt, but from a balanced diet, our drinkable yogurt formulation may be helpful to increase the intake of minerals when combined with other sources of minerals, which, in turn, may render health benefits.
3.3. Physicochemical Analyses
Yogurt 0% (with pulp but devoid of PFF) and those with pulp and different concentration of PFF (2–8%) showed a lower pH than plain drinkable yogurt (Table 1
), which is explained by the pH of the pulp (3.20) and PFF (3.72). At the first day of storage (Table 1
), no differences were observed among drinkable yogurts with PFF (2–8%). During storage (Table 2
), the pH values recorded were from 3.99 to 4.57 for the drinkable yogurt samples. This decrease was already expected and probably occurred due to the symbiotic relation between Streptococcus thermophilus
and Lactobacillus delbrueckii
, which are homofermentative bacteria that present high metabolic activity [11
]. The same behavior was observed by other studies that reported a pH range from 4.0 to 4.6 in different drinkable yogurt samples [11
Regarding the syneresis, it was observed that the plain drinkable yogurt and the drinkable yogurt with 8% PFF showed the highest and the lowest syneresis percentages, respectively (Table 2
). In general, the syneresis increased during the storage time, but there was no difference between the 21st and 28th day of storage (except for the drinkable yogurts with 2% and 4% of PFF). During the storage, pH values decreased while the syneresis values increased (Table 2
). Similar observations were reported by García-Perez et al. [39
] and Hashim et al. [41
], which was expected because as the milk is acidified, the charges on caseins decrease, which weakens the electrostatic forces holding micelles together, and weakens the steric stabilization. This decreases the attractive force between micelles, which become ‘adhesive’ or ‘sticky’ and forms a weak particle gel with serum separation [42
Treatments with higher PFF concentrations remained more stable and lowered syneresis due to the high fiber content and its water-holding capacity that absorbed the whey released by the gel structure [39
]. According to Ramirez-Santiago et al. [43
], the addition of 1% yam soluble fiber (YSF) extracted from Pachyrhizus erosus
L. reduced the syneresis of stirred yogurt and rendered a more acceptable mouthfeel compared to the control yogurt (devoid of YSF). Tseng and Zhao [40
] used wine grape pomace (WGP) as a source of dietary fiber and verified that the addition of 1% and 2% WGP rendered yogurts that were more stable regarding syneresis during three weeks of storage.
Color is one of the most important quality parameters for the consumer acceptance of foods [10
]. Drinkable yogurts became darker (Table 1
) with the addition of PFF as demonstrated by the lightness value (L). As for the chroma (C), drinkable yogurts with PFF showed lower values than the samples devoid of PFF (yogurt 0%). Likewise, the hue angle (h) value decreased with a concurrent increase in the concentration of PFF. This parameter indicated that these samples presented yellow tonality. Yogurt 0% and yogurt 8% showed, respectively, the strongest and weakest yellow intensities and this demonstrated that even at low concentrations PFF affected the color of the samples. The color of yogurt depends on its pH [39
]. It can be observed that parameters, such as L and h, had a positive correlation with pH (r = 0.883 and r = 0.857, respectively), while the parameter C presented negative correlation (r = −0.734). In all cases, it was noted that PFF addition influenced drinkable yogurt color. Other studies [21
] that used by-products in yogurt formulations also commented on changes in the color parameters of their products.
Concerning the drinkable yogurt viscosity, it was observed that the drinkable yogurt with pulp, but without PFF (yogurt 0%), present the lowest viscosity value while the yogurt 8% demonstrated the highest one. The addition of PFF increased the viscosity of the samples due to the high content of fiber (64.0%). A similar result was observed by Arioui et al. [11
] in yogurt fortified with Citrus sinensis
peels and the best viscosity value was obtained with 0.6% of pectin. Aportela-Palacios et al. [44
] stated that wheat bran is a great option as a fiber source to increase the viscosity and consistency of yogurts. Despite the effects of fiber addition on the viscosity, the texture of drinkable yogurt can also be affected by other factors such as casein content of the milk, firmness of the gel before stirring, the intensity of stirring, and syneresis [45
3.4. Microbiological Evaluation
counts were < 3 MNP/g for all treatments, which is within the established limits (10 MNP/g) [46
]. During storage, the lactic acid bacteria count was monitored (Table 3
). Plain drinkable yogurt had the highest number of lactic acid bacteria, but PFF addition was not correlated with the lactic acid bacteria count (r = −0.0269); therefore, the flour addition did not impair the drinkable yogurt shelf life. Perina et al. [22
] also reported on the use of passion fruit by-products in yogurt formulation and they did not observe any influence of PFF on lactic acid bacteria.
A decrease in lactic acid bacteria count was observed during storage, which may be related to the low pH post-acidification. Streptococcus thermophilus
and Lactobacillus delbrueckii
live in a mutual stimulation that is related to the growth, acidification, and the production of aromatic compounds. After fermentation, organic acid accumulation (e.g., lactic and acetic acid) occurs. Organic acids are powerful antimicrobial agents; therefore, they may affect the survival of microorganisms [47
]. Additionally, L. bulgaricus
is more sensitive to storage than S
, and it usually presents a large decrease in its viable cells [1
]. For yogurt, the sum of microorganisms in the starter culture must be at least 107
]. As demonstrated in Table 3
, all samples had adequate amounts of viable lactic acid bacteria until the 21st day of storage. Therefore, the present formulations are satisfactory for three weeks, which defines the product shelf life.
Although some microbiological aspects of yogurts fortified with passion fruit by-products have already been investigated [1
] no attention has been paid to the presence of yeast and mold growing, which may also impair the product shelf life. The present study demonstrated that the addition of passion PFF may contribute to the growth of yeast and mold in drinkable yogurt (Table 3
). Contamination by yeast and molds is a major problem faced by the dairy industry. According to Ledenbach and Marshall [49
], yeast and mold can promote changes in sensory characteristics of yogurt, like yeasty and fermented off-flavors and a gassy appearance. The authors stated that yogurts elaborated under conditions of good manufacturing practices should contain no more than 10 yeast cells and should present a shelf life of 3–4 weeks at 5 °C. For the present drinkable yogurt formulation, yeast and mold counts were higher than this recommendation after 21 days of storage.
In order to increase the drinkable yogurt shelf life, it is suggested that the use of plant food by-products should come along with additional processes, such as gamma-irradiation to ensure microbiological decontamination; or drying, that may be helpful to reduce the water activity of the product, making the growth of spoilage and pathogenic microorganisms more difficult [50
]. In addition, the use of other technologies, such as UV pasteurization, is also recommended to increase the shelf life of beverages [51
3.5. Sensory Evaluation
The sensory acceptance test demonstrated that the drinkable yogurt with the highest PFF concentration (yogurt 8%) generated the lowest score given by the panelists (Figure 1
A). However, concentrations up to 4% received scores higher than five, indicating that panelists accepted the sample. According to the panelists, the yogurt 0% and 2% presented a pleasant and light flavor with a good texture and appearance, whereas the yogurt 8% was described as bitter, showing a heterogeneous texture with perceptible granules. Furthermore, it is important to highlight that the yogurt 8% presented the lowest moisture content (Table 1
), which may interfere in its texture and be a rejection factor for the consumer [52
]. Many panelists stated they would consume the yogurt 2% due to its association with a healthier diet because they perceived and liked the presence of fiber in the drinkable yogurt formulation. According to Gahruie et al. [53
], despite the fiber-related health benefits, it is important to note that consumers do not usually accept formulations with more than 3% fiber.
Sensory descriptive analysis was also performed to provide the sensory profile of the drinkable yogurt. For this test, drinkable yogurts with pulp and PFF concentrations of 0, 2, and 4% of PFF were used because they received higher acceptance (scores > 5 in the acceptance test). The panelists described their perceptions about the product. In-group consensus descriptive terms, definitions, and references were defined for the following attributes: appearance (colorful, attractive, bright, and homogeneous), aroma (expected and sweet), flavor (expected, sweet, and bitter), and texture (expected, consistent, and viscous) (Table 4
). After the training sessions, the panelists participated in the final stage of the descriptive analysis. In many attributes, the drinkable yogurts with PFF (yogurt 2% and 4%) presented significant differences compared to the drinkable yogurt devoid of PFF (yogurt 0%) (Figure 1
B). Therefore, the PFF was responsible for the different attributes that were not exhibited in the traditional product (yogurt 0%), which may be an innovation for the dairy products market.
A brownish color was observed in drinkable yogurts with PFF, but the darkening of the samples did not compromise the attribute ‘attractive’, which received satisfactory scores (all above 8.0) for all treatments. No differences were found in the ‘brightness’ attribute between the drinkable yogurts manufactured with 2% and 4% PFF and these samples even presented higher scores for this attribute compared to the control.
The drinkable yogurts with PFF presented a high intensity of expected aroma, expected flavor, and expected texture, and the drinkable yogurts (2% and 4%) did not present statistical differences. The addition of PFF increased the intensity of sweet aroma and flavor, and the yogurt 4% showed the highest score for both attributes (4.52 and 4.63, respectively). The panelists have also identified a slightly bitter taste in drinkable yogurts, which may be explained by the presence of phenolic compounds in passion fruit mesocarp [54
]; therefore, the samples presented low scores for this attribute (less than 2.40). Significant differences were found among all treatments, and the scores for bitter taste were proportional to the PFF concentration. In a yogurt formulation developed by Perina et al. [22
], panelists noted a bitter taste for yogurt that may be considered a driver of disliking when in excess, and the authors associated this bitter taste to the addition of strawberry pulp and unflavored passion fruit peel-powder.
No differences were observed on the consistency and viscosity between yogurt 2% and 4%, and both presented higher scores as compared with yogurt 0%. This fact is associated with the higher fiber contents of such samples, which may be responsible for the thick and viscous texture. Perina et al. [22
] did not find significant differences between the texture of yogurts with passion fruit peel and seeds flour and the control. However, Raju and Pal [21
] observed that yogurt fortified with 1.5% of soy fiber or oat fiber present lower scores for texture than the control.